The specific association of biological macromolecules is fundamental to all living systems. The recognition and combination of two such molecules depends on the three-dimensional structures of each of them. We propose to determine by single crystal x-ray crystallography the tertiary structures of macromolecules which combine specifically with other macromolecules. We have chose to focus on two different problems of macromolecule recognition and association. One is the agglutination of certain cell surfaces by plant lectins. This study has led to the determination of the structure of wheat germ agglutinin. Current and proposed work is directed towarding finding the sites on this molecule which bind to groups of sugars attached to cell surfaces. An unusual internal structural redundancy has already suggested the orientations which agglutinated groups bear to one another. The second problem which we are studying is the nature of the recognition of specific tRNA's as substrates in enzyme-catalyzed reactions. We have determined the structure of yeast tRNA Gly to a resolution of approximately 4 A and found significant difference between this structure and that of yeast tRNA Phe. We do not believe that this difference in structure is instrinsic to the tRNA molecules, but reflects the effect of different solvent environments on them. This second tRNA conformation observed in tRNA Gly suggests interesting hypotheses for the specificity of such reactions as aminoacylation and we are beginning to test them as well as to improve our crystal structure and extend it to the same tRNA Gly species in different solvent conditions.